1. Field of the Invention
The present invention relates to a hologram recording medium and a hologram record-reproduction device using a principle of holography.
2. Description of the Related Art
A hologram recording and reproducing apparatus is known in which a laser beam is divided into two, one of them is modulated with information that is to be recorded with a spatial optical modulator and this information beam is caused to fall on an optical recording medium (referred to hereinbelow as a hologram recording medium) having a recording layer of a hologram substance (referred to hereinbelow as a hologram recording layer), whereas the other beam is caused to fall as a reference beam at the prescribed angle of incidence with respect to the information beam in an information beam illumination position on the hologram recording layer of the hologram (interference pattern) recording medium and the hologram generated by the information beam and reference beam is recorded on the hologram recording layer.
With the hologram recording method, a plurality of holograms can be recorded (angular multiplex recording) in the same recording position of the hologram recording medium by changing the angle of incidence of the reference beam. Therefore, the capacity which is more than several tens of times greater than that of the CD (Compact Disc) or DVD (Digital Versatile Disc) can be recorded and reproduced with the hologram recording and reproduction suggested in prior art.
Further, in the above-described hologram recording and reproduction device, a diffraction beam (information beam) is generated from the hologram medium by causing he reference beam to fall at the same angle and phase-map of incidence as during recording on the hologram recording medium, this diffraction beam is received with a pickup element, and the desired information is reproduced by conducting the prescribed signal processing of the received optical signal.
The hologram recording system enables large-capacity recording by angular multiplex recording, but when the diffraction beam (information beam) is generated by causing the reference beam to fall on the hologram recorded on the hologram recording medium, the intensity (diffraction efficiency) of the diffraction beam is known to follow the second power distribution of a sinc function, as shown in FIG. 19, and the specific feature of such a process is that the allowed range (margin) of both the shift of the angle θ of incidence of the reference beam during recording and reproduction and the shift of the wavelength λ of the light source are very narrow (angular selectivity and wavelength selectivity are extremely high).
In FIG. 19, the intensity of the diffraction beam (a value normalized with respect to a maximum value taken as “1.0”) is plotted against the ordinate, and the shift quantity Δθ of the angle of incidence of the reference beam or the shift quantity Δλ of the wavelength of the light source is plotted against the abscissa. The position “0” on the abscissa is the position in which the angle of incidence of the reference beam or the wavelength of the light source match those during recording and in which the shift is zero. At this time, the intensity of the diffraction beam assumes a maximum value. When the shift quantity Δθ of the angle of incidence of the reference beam is plotted against the abscissa, FIG. 19 shows the relationship between the intensity of the diffraction beam and the shift quantity Δθ of the angle of incidence of the reference beam, and when the shift quantity Δλ of the wavelength of the light source is plotted against the abscissa, FIG. 19 shows the relationship between the intensity of the diffraction beam and the shift quantity Δλ of the wavelength of the light source.
For example, when recording is conducted on a hologram recording medium with a thickness of the hologram substance of 200 μm by using a laser beam with a wavelength of the light source of 405 nm at an angle of incidence of the reference beam of 30°, then the shift quantity Δλ of the wavelength of the light source in point (a) and point (b) in which the intensity of the diffraction beam is “0” will be about ±6.1 nm and the shift quantity Δθ the angle of incidence of the reference beam will be about ±7 min. Therefore, if the margin of the diffraction beam decrease during reproduction of information recorded on the hologram recording medium is taken as “0.5” (3 dB down), then the shift quantity Δλ3dB (shift quantity Δλ of 3 dB down) of the wavelength of the light source at which the intensity of the diffraction beam becomes “0.5” is about ±3 nm and this value becomes the margin for the shift of the wavelength of the light source. Furthermore, the angle of incidence Δθ3dB of the reference beam at which the intensity of the diffraction beam becomes “0.5” is about ±3.5 minutes and this value becomes the margin for the shift of the angle of incidence of the reference beam.
Thus, with the hologram recording method, the margins for the shift of the wavelength of the light source and the shift of the angle of incidence of the reference beam are extremely narrow (severe conditions relating to angle selectivity or wavelength selectivity). Therefore, because of such conditions it is difficult to realize a replaceable and portable hologram recording medium that uses optical disks such as CD or DVD.
Accordingly, a technology for increasing the stability and reliability of recording and reproduction by improving methods for recording information on a hologram recording medium have been heretofore suggested.
For example, Japanese Patent Application Laid-open No. H11-16374 describes a hologram recording method by which the decrease in diffraction efficiency of information beam is reduced as much as possible and reliability of reproduction is increased by recording on a hologram recording medium the information that is to be recorded and also the information relating to an angle of incidence of the reference beam (referred to hereinbelow as “recording angle”) during recording of the aforementioned information, reading the information relating to the recording angle during reproduction, and correcting the angle of incidence of the reference beam (angle of incidence with respect to the hologram recording medium during reproduction) based on this information.
Further, Japanese Patent Application Laid-open No. 2002-216359 describes a technology using the fact that the position of a reference beam projected on the pickup surface of a pickup element in which a multiplicity of light receiving elements are arranged in the form of a grid is shifted according to the shift quantity of the wavelength of the laser beam when such occurs, this technology comprising the steps of employing a wavelength-variable coherent light source as a light source, using optical received signals of the light-receiving elements in four corners of the pickup element during reproduction, detecting the shift quantity of the protection position of the reference beam on the pickup surface of the pickup element and correcting the wavelength of the laser beam outputted from the wavelength-variable coherent light source based on this shift quantity.
The aforementioned Japanese Patent Applications Laid-open Nos. H11-16374 and 2002-216359 described the realization of a portable and replaceable hologram recording medium with an optical disk configuration by using servo technology of focusing and tracking of optical disks such as CD and DVD. However, in the case of a hologram recording system, as described hereinabove, the margins for the shift of the wavelength of the light source and the shift of the angle of incidence of the reference beam are extremely narrow and cannot be considered as sufficient for the margins in the servo technology of focusing and tracking of optical disks such as CD and DVD.
For this reason, in the servo technology of focusing and tracking of optical disks such as CD and DVD, it is difficult to conduct reliably the servo control of focusing and tracking of a hologram recording medium of an optical disk type and data hardly can be reordered and reproduced with good stability.
Further, when digital data such as image data or text data are recorded on a recording medium, the recording medium is generally divided into a plurality of sectors and image data are divided and recorded in a respective plurality of sectors (unit recording areas). On the other hand, management data relating to such items as which image data has been recorded on the recording medium and which sector has been used for recording the image data are also recorded, for example, as represented by a FAT (File Allocation Table).
When the files recorded on the disk are read, first, it is necessary to read the file management information with good reliability, and unless the file management information is read with high stability and reliability, even if the disk capacity is increased, the effect attained thereby is reduced by half. Therefore, the reliability of reading and writing the management data for data being recorded is more important than that of the data being recorded. Further, even if servo control of focusing and tracking of the hologram recording medium during recording and reproduction has been realized, the device actually cannot function as a hologram recording and reproduction device unless the management data are read reliably in the subsequent recording or reproduction processing of the management data.
In the case of a hologram recording system, margins for the shift of the wavelength of the light source and the shift of the angle of incidence of the reference beam are extremely narrow and the diffraction beam is essentially difficult to reproduce with good stability and reliability from a hologram recorded on the hologram recording medium. Therefore, it is desirable that the margins for the shift of the wavelength of the light source and the shift of the angle of incidence of the reference beam during recording and reproduction of the management information of files be relaxed with respect to the margins for the shift of the wavelength of the light source and the shift of the angle of incidence of the reference beam of the files.
The hologram recording and reproduction devices described in the aforementioned Japanese Patent Applications Laid-open Nos. H11-16374 and 2002-216359 employ servo control of focusing and tracking of the hologram recording medium of an optical disk type by using servo control technology of focusing and tracking employed in optical disks such as CD and DVD. Therefore, the stability and reliability of data recording and reproduction cab hardly be considered as sufficient.
Furthermore, the technology described in Japanese Patent Application Laid-open No. H11-16374 minimizes the angle of incidence of the reference beam by correcting the angle of incidence of the reference beam during reproduction, and the margin for the shift of the wavelength of the light source or the shift of the angle of incidence of the reference beam during recording and reproduction does not vary between files and management data for the files. Furthermore, the technology described in Japanese Patent Application Laid-open No. 2002-216359 minimizes the shift of the wavelength of the light source by changing the generation frequency of the light source during reproduction. In this case, too, the margin for the shift of the wavelength of the light source or the shift of the angle of incidence of the reference beam during recording and reproduction does not vary between files and management data for the files. Therefore, with both methods there may be cases where the files cannot be opened because file management information is not read.
In particular, if a system is used in which hologram recording is conducted by rotating a disk, the margin for the shift of the angle of incidence of the reference beam becomes even more strict because of eccentricity or in-plane wobbling caused by very small tilting of the disk, and the file management information is difficult to read. For this reason, a long time is required for reading the file management information and in some cases this information cannot be read at all. Accordingly, recording and reproduction processing is extremely difficult to conduct at a high rate and stability. Further, if the thickness of the hologram medium is increased to make a transition to ultrahigh capacity of recording by angular multiplexing, then because the margin shown in FIG. 19 is inversely proportional to the thickness of the hologram medium, this margin is further narrowed. As a result, a certain limitation is inevitably placed on the transition to ultrahigh capacity by increasing the thickness of the hologram medium.